Housing units formed in a factory are generally much more economical than units built on site because they enjoy the advantages of mass production. My U.S. Pat. No. 3,461,663 discloses a prefabricated building structure formed of a three-dimensional rectangular truss utilizing steel framing members. Modules may be erected in a factory and shipped to a building site where a number of modules may be joined together to form a larger structure.
Independently it has been proposed to join a plurality of modules to form multiple unit housing structures. See for example U.S. Pat. No. 6,651,393.